What if there was a virus that could not only kill you, but actually compel you to go out and infect as many of your friends as possible? Such a virus exists — in caterpillars at least — and now a team of researchers have identified the viral gene responsible for the bizarre behavior it engenders.

The life of a gypsy moth caterpillar is a simple one. Every night, the caterpillar climbs high into the trees to feed on leaves. By day, however, it returns to the ground to avoid predators. Unless they're infected with a baculovirus, that is.

Baculoviruses are used to control gypsy moth populations. They do so by wasting away at a caterpillar's insides, then manipulating the caterpillar's behavior in such a way that it spends what little strength it has left making its way high into the trees — even in the day time. But why would it do such a thing?

The better to disperse virus, of course. See the baculovirus doesn't just compel the caterpillar to climb high into the treetops, it also uses the caterpillar's body to reproduce. As it does so, the caterpillar dies, liquefies, and then disperses its virus-y self all over the underlying folliage, infecting any other caterpillars it may come in contact with.

A research team led by entomologist Kelli Hoover hypothesized that the behavior of infected caterpillars might be connected to a gene in the virus called egt. The gene egt codes for an enzyme, EGT, that inactivates the hormone in these caterpillars that triggers molting. Because caterpillars usually descend from the treetops to molt on the ground, the researchers reasoned that the bugs were staying high in the trees to die because EGT was blocking the molting hormone.

To test this hypothesis, Hoover and her team generated versions of the virus missing the egt gene, and infected caterpillars with both the modified and unmodified (aka "wild type") virus. Caterpillars were then placed in tall experimental bottles, where they were observed right up until their liquidy, liquidy death.

Both wild type viruses containing the intact egt gene produced larval death at elevated positions, whereas deletion of egt eliminated this behavior. Moreover, rescue of egt restored climbing behavior. Larvae infected with wild-type virus or egt-deletion viral strains behaved similarly during early and middle phases of viral infection. However, during the late stage of infection egt-deletion–infected insects returned to the container bottoms and died there, whereas wild-type infected larvae stayed in elevated positions and died.

The researchers say that their findings provide solid evidence of a genetic basis for modified caterpillar behavior.

"One of the best ways to control complex behavior is to manipulate hormones," said Hoover. "In this case we've found that that the gene also somehow induces the caterpillars to go to just the right location to enhance transmission of the virus to new hosts."